Spin-polarized electrons in the electron source are photoemitted from negative electron affinity (NEA) GaAs by exciting transitions with circularly-polarized infrared (IR) radiation [1]}, [2]}, [3]}, [4]}. The angular momentum conservation of either right- or left-helicity is used to select the orientation of the spin polarization of the electron beam. For the excitation of the photoelectrons, we use a diode laser with a wavelength of 830 nm and 30 mW of CW power. The laser intensity on the photocathode as well as the helicity of the laser light can be controlled by two liquid crystal (LC) retarders in alternating sequence with two Glan Thomson polarizers. The first LC retarder is located between two crossed Glan Thomson polarizers and allows the adjustment of the transmission by its variable birefringence. The second LC retarder receives linearly polarized light from the second Glan Thomson polarizer. By invoking an adjustable phase shift, which is either set to quarter wave or three quarter wave delay, a positive or negative helicity is imposed on the outgoing laser beam.
